The semiconductor integrated circuit (IC) industry has experienced rapid growth. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs, and, for these advances to be realized, similar developments in IC manufacturing are needed.
For example, traditional transmissive photolithography is being supplemented or replaced with reflective photolithography. ICs are typically assembled by layering features on a semiconductor substrate using a set of photolithographic masks. Transmissive masks have patterns formed by transmissive regions. During photolithographic exposure, radiation, such as ultraviolet light, passes through the transmissive regions of the mask before striking a photoresist coating on the substrate. The mask transfers the pattern onto the photoresist. In contrast, a reflective mask includes reflective and non-reflective regions. During exposure, the light reflected off the mask is used to form the pattern on the substrate. After either type of exposure, the photoresist is selectively removed to reveal the pattern. The substrate then undergoes processing steps that take advantage of the shape of the remaining photoresist to create circuit features on the substrate. When the processing steps are complete, photoresist is reapplied and substrate is exposed using the next mask. In this way, features are layered to produce the final circuit.
Reflective masks are advantageous in many applications because they can be used in conjunction with relatively higher frequency radiation such as extreme ultraviolet (EUV) radiation. EUV radiation forms more precise patterns and smaller features than conventional UV radiation, but has proven challenging to use in lithography. For example, most mask materials block EUV radiation, making it difficult to manufacture a suitable transmissive mask. In contrast, reflective masks are more easily manufactured and tuned for EUV environments. For this reason and others, reflective masks and reflective lithography have delivered positive results but present challenges as well.